Since its beginnings in the late 1940's, the offshore petroleum industry has been steadily moving into progressively deeper waters. Until recently, offshore petroleum drilling and producing operations typically have been conducted from rigid, bottom-founded offshore structures such as conventional steel jacket structures or concrete or steel gravity structures. However, as described below, water depths of interest to the offshore petroleum industry have now increased to the point where such rigid, bottom-founded structures are no longer technically or economically feasible.
An offshore structure must be designed to withstand not only the relatively infrequent impacts of very large waves caused by severe storms, but also the cumulative effect of repeated impacts of smaller waves which are present under most sea states. These smaller waves are typically random in nature. However, it has been found that the wave periods of these smaller waves generally fall between about 6 seconds and about 20 seconds. Such waves are likely to contain significant amounts of energy.
When a wave impacts on an offshore structure, it causes a dynamic flexural vibration in the structure generally known as a wave dynamic response. If the flexural vibration period of the structure falls within the range of wave periods likely to contain significant amounts of energy, (i.e., 6 seconds to 20 seconds), the structure will resonate under certain conditions. Resonance of the structure is likely to impose excessive forces on the structure and may result in fatigue damage. Accordingly, offshore structures are designed so that the flexural vibration period of the structure falls outside the range of wave periods likely to contain significant amounts of energy. Rigid, bottom-founded structures are typically designed so that the flexural vibration period of the structure is less than about 6 or 7 seconds, depending on the location of the structure.
The wave dynamic response of a rigid, bottom-founded structure may be characterized as a lateral vibration of a beam having one end fixed and the other end free. Accordingly, for a structure having a given flexural stiffness and a given distribution of weight along its length, the flexural vibration period of the structure is proportional to the height of the structure (depth of the water) squared. Therefore, as water depth increases, the flexural stiffness of a rigid, bottom-founded structure must be increased so as to maintain the flexural vibration period within acceptable limits.
The design of a rigid, bottom-founded structure begins to be dominated by wave dynamic response in water depths of about 800 to 1,000 feet. Past experience has shown that once wave dynamic response begins to dominate the design, the structural steel tonnage, and hence the cost, required to maintain the flexural vibration period of the structure within acceptable limits increases very rapidly. Beyond a water depth of about 1,000 feet, the steel tonnage and associated costs for a rigid, bottom-founded structure increase so rapidly that an economic limit is soon reached, even given the most favorable economic conditions.
The problem outlined above has resulted in the development of new types of offshore structures generally known as "compliant towers". Compliant towers are bottom-founded structures that do not rigidly resist environmental forces. Rather, a compliant tower is designed to yield to the environment in a controlled manner. Basically, the tower is allowed to oscillate a few degrees from vertical in response to the applied force. This oscillation creates an inertial restoring force which opposes the applied force.
One such compliant tower is the "guyed tower". Basically, a guyed tower is a trussed frame of generally uniform cross-section that extends from the bottom of the body of water upwardly to a deck supported above the water surface. The tower is held upright by multiple guy lines which are spaced about its periphery. The guy lines permit the tower to pivot a few degrees from vertical about its base in response to surface wind, wave, or current forces, thereby creating inertial forces which counteract the applied forces. The guy lines optionally may include intermediate clump weights and the tower optionally may include buoyancy tanks, both of which aid in restoring the tower to a vertical position. See generally, Finn, L. D., "A New Deep-Water Platform--The Guyed Tower", Journal of Petroleum Technology, April 1978, pp 537-544 (first presented at the 8th Annual Offshore Technology Conference held in Houston, Tex., May 3-6, 1976, OTC Paper No. 2688).
A second type of compliant tower is the "buoyant tower". Basically, a buoyant tower is similar to a guyed tower except that no guy lines are used. The entire restoring force for the tower is provided by large buoyancy tanks attached to the tower, preferably at or near the surface of the body of water. See, for example, the buoyant tower illustrated in U.S. Pat. No. 3,636,716 issued Jan. 25, 1972 to Castellanos.
As described above, the primary response of a compliant tower to environmental forces is oscillation a few degrees from vertical about its base in the manner of an inverted pendulum, with either or both of guy lines and buoyancy tanks providing the restoring force. The guy lines and the water surrounding the tower provide a sufficient amount of damping to quickly damp off the oscillation. The guy lines and buoyancy tanks are typically designed so that the oscillation period of the tower in response to environmental forces is greater than about 20 seconds. Thus, the oscillation period falls outside the range of wave periods likely to contain significant amounts of energy. However, as described below, compliant towers are also subject to the problem of lateral vibration induced by the impact of random surface waves.
A compliant tower may be characterized as a beam having one pinned end, one free end, and a variable restoring force applied at and perpendicular to the free end. When a wave impacts on a compliant tower, it causes both the rigid oscillation previously described and a dynamic flexural vibration. Thus, at the same time, the tower oscillates in the manner of an inverted pendulum and vibrates in the manner of a bowstring. As with rigid, bottom-founded structures, the flexural vibration period of a compliant tower must be less than about 6 or 7 seconds in order to prevent resonance with the waves.
Due to the different types of end restraints (i.e., pinned versus fixed), the flexural vibration period of a compliant tower is less than about one-fourth of the flexural vibration period of a rigid, bottom-founded structure having the same length, weight distribution, and flexural stiffness. Therefore, compliant towers may be used in water depths substantially greater than those for which rigid, bottom-founded structures are practical. However, the design of a compliant tower begins to be dominated by flexural vibration (wave dynamic response) in water depths of about 1,800 to 2,000 feet. Beyond those depths, the steel tonnage and associated costs required to maintain the flexural vibration period of a compliant tower within acceptable limits increase so rapidly that a point is soon reached beyond which compliant towers are no longer economically practical.
Hydrocarbon reservoirs of interest to the offshore petroleum industry have been located in water depths substantially greater than 2,000 feet. Due to the flexural vibration problem described above, neither conventional rigid, bottom-founded structures nor the newer compliant towers may be economically used to produce hydrocarbons from these deep water reservoirs. Accordingly, the need exists for an offshore structure which can be economically used to produce hydrocarbons in water depths greater than 2,000 feet.
The hybrid offshore structure of the present invention satisfies the need outlined above by utilizing a compliant upper section pivotally mounted to the top of a substantially rigid lower section. The lower section extends upwardly from the bottom of the body of water to a pivot point located intermediate the bottom and the surface of the body of water. The location of the pivot point is selected so as to substantially minimize the weight of the structure while maintaining the flexural vibration period of the structure within acceptable limits. Typically, the pivot point would be located above the bottom a distance of between about 10 percent and about 50 percent of the total depth of the body of water. As hereinafter described in greater detail, for a limited range of pivot heights, the weight of steel required to maintain the flexural vibration period of a hybrid structure within acceptable limits may be significantly less than that required for either a rigid, bottom-founded structure of a compliant tower in the same water depth.
Previous offshore structures have utilized a compliant upper section pivotally mounted to the top of a base section. See, for example, the structures disclosed in U.S. Pat. No. 3,522,709 issued Aug. 4, 1970 to Vilain, U.S. Pat. No. 3,553,969 issued Jan. 12, 1971 to Chamberlin et al., U.S. Pat. No. 3,636,716 issued Jan. 25, 1972 to Castellanos, U.S. Pat. No. 3,670,515 issued June 20, 1972 to Lloyd, U.S. Pat. No. 3,735,597 issued May 29, 1973 to Guy, U.S. Pat. No. 4,231,682 issued Nov. 4, 1980 to Tuson, and U.S. Pat. No. 4,273,470 issued June 16, 1981 to Blomsma et al. Generally, the primary purpose of the base section in each of these structures is simply to provide an appropriate foundation for the pivot. None of the patents specifies the height of the base section or attaches any particular significance thereto. Further, none of the patents contains any teachings that use of a lower section having a height of between about 10 percent and about 50 percent of the total depth of the body of water may reduce the weight (and cost) of the structure while maintaining the flexural vibration period of the structure within acceptable limits.
One previous offshore structure which utilizes a base section having a non-negligible height is illustrated in FIG. 5 of U.S. Pat. No. 3,768,268 issued Oct. 30, 1973 to Laffont et al. In Laffont et al. the pivot point is located approximately 300 to 600 feet below the surface of the body of water since below that depth the wave swell has relatively little effect. Thus, in water depths greater than 2,000 feet, the structure disclosed in Laffont et al. would have a pivot height of more than 70 percent of the total water depth. As will be apparent from the following discussion of the present invention, for a structure such as the one illustrated in FIG. 5 of Laffont et al, a pivot height of 70 percent of the water depth would likely result in a structure having a considerably higher flexural vibration period than a compliant tower in the same water depth and having comparable stiffness and weight distribution.